Abstract Kimberlites are the deepest sourced magmas emplaced at the Earth's surface. They provide a “window” into the processes at the base of the subcratonic mantle. A better understanding of the origin, composition, and emplacement mechanisms of kimberlites is hampered by uncertainties in the contents of the two main volatiles, H2O and CO2. Diamond dissolution in H2O and in CO2 fluids produces distinct resorption features offering an opportunity to determine the composition of the magmatic fluid in kimberlites. Here we examined the relationship between H2O:CO2 ratio of the fluid and the style of diamond resorption by conducting experiments in C-O-H fluid saturated with silicates with variable H2O:CO2 ratios at the conditions of kimberlite ascent of 1–3 GPa and 1150–1350 °C. Our results showed that the geometry of etch pits on diamond and the resorption style evolve consistently as bulk CO2 content of the fluid changes from 0 to 50 to 50–90 and 90–100 mol%. The fluid composition at the run conditions was monitored by entrapment of synthetic fluid inclusions in olivine and quartz. The inclusions demonstrated the existence of a fluid miscibility gap at 1–3 GPa and 1250 °C with two fluid endmembers, an aqueous and a carbonic phase, which H2O:CO2 ratio at 1 GPa determined with confocal Raman microscopy is (H2O)0.62(CO2)0.38 and (H2O)0.12(CO2)0.88 respectively. Hence, diamond resorption morphology depends on the proportions of the end-member aqueous and carbonic fluids, which vary with the bulk composition of the fluid. The different density and ability of aqueous and carbonic fluids to dissolve silicates (olivine) would promote their separation in the rising magma column. Concentration of the lower density aqueous fluid towards the tip of the propagating dyke would facilitate more efficient fracturing of the country rocks and faster ascent of the kimberlite magma causing explosive eruption. We propose that preferential attachment of aqueous fluid bubbles would help to increase the buoyancy of olivine xenocrysts and possibly diamond in the kimberlite magma offering a mechanism for transporting the heavy mantle cargo.

中文翻译：

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金伯利岩中的金刚石吸收和 COH 流体的不混溶性：来自 H2O – CO2 – SiO2 – MgO – CaO 系统在 1–3 GPa 中的实验证据

摘要 金伯利岩是地球表面最深的岩浆。它们为亚克拉通地幔底部的过程提供了一个“窗口”。两种主要挥发物 H2O 和 CO2 含量的不确定性阻碍了对金伯利岩的起源、组成和侵位机制的更好理解。钻石在 H2O 和 CO2 流体中的溶解会产生明显的再吸收特征，这为确定金伯利岩中岩浆流体的成分提供了机会。在这里，我们通过在金伯利岩上升 1–3 GPa 和 1150–1350 ° 的条件下，在饱和了硅酸盐的 COH 流体中进行实验，研究了流体的 H2O:CO2 比与金刚石吸收类型之间的关系，其中 H2O:CO2 比率可变C。我们的结果表明，随着流体的整体 CO2 含量从 0 到 50 到 50-90 和 90-100 mol% 的变化，金刚石上蚀刻坑的几何形状和再吸收方式一致地演变。通过在橄榄石和石英中截留合成流体包裹体来监测运行条件下的流体成分。包裹体证明在 1-3 GPa 和 1250 °C 存在流体混溶间隙，具有两个流体端元，水相和碳相，共聚焦拉曼显微镜测定的 1 GPa 时的 H2O:CO2 比为 (H2O)0.62( CO2)0.38 和 (H2O)0.12(CO2)0.88 分别。因此，金刚石再吸收形态取决于端元含水流体和含碳流体的比例，这些比例随流体的整体组成而变化。含水流体和碳酸流体溶解硅酸盐（橄榄石）的不同密度和能力将促进它们在上升的岩浆柱中分离。将密度较低的含水流体向扩展岩脉的尖端集中，将有助于更有效地压裂围岩，并使金伯利岩浆更快上升，从而导致爆发性喷发。我们提出，水性流体气泡的优先附着将有助于增加金伯利岩浆中橄榄石异晶和可能的钻石的浮力，从而提供运输重型地幔货物的机制。将密度较低的含水流体向扩展岩脉的尖端集中，将有助于更有效地压裂围岩，并使金伯利岩浆更快上升，从而导致爆发性喷发。我们提出，水性流体气泡的优先附着将有助于增加金伯利岩浆中橄榄石异晶和可能的钻石的浮力，从而提供运输重型地幔货物的机制。将密度较低的含水流体向扩展岩脉的尖端集中，将有助于更有效地压裂围岩，并使金伯利岩浆更快上升，从而导致爆发性喷发。我们提出，水性流体气泡的优先附着将有助于增加金伯利岩浆中橄榄石异晶和可能的钻石的浮力，从而提供运输重型地幔货物的机制。